Mobile phones and similar devices often feature several wireless subsystems such as Bluetooth, Wifi and/or GPS, as well as the cellular subsystem (a.k.a. baseband). Each wireless subsystem depends on a stable local reference clock to synchronize and communicate with peers, satellites or the network infrastructure.
On a typical design, the cellular subsystem and the wireless group comprising Bluetooth, GPS and Wifi subsystems both derive their reference clock from their own crystal oscillator. These oscillators normally deliver a reference clock that is close to perfect but temperature variations, aging and other factors cause fluctuations that require some kind of adjustment scheme to meet application-specific accuracy constraints. Adjustment is usually done by matching the local reference clock against a known clock that is inferred from the wireless signal received by the respective subsystem. It takes some effort (in terms of time and/or power) to adjust the reference clock and therefore any way to speed this process up is beneficial to the system.